These new buses meet the latest emissions requirements for highway diesel engines. My bus is equipped with a Diesel Particulate Filter (DPF) that also uses a Diesel Exhaust Fluid (DEF) system. DEF is automatically injected into the exhaust system to help collect and trap harmful particles in the diesel fumes. These particles are then deposited in the DPF and burned off over time. A separate tank holds several gallons of DEF and is refilled when needed. A gauge on the dash tells you when your low on DEF.